1. Field of the Invention
The present invention relates to automatic calibration of sensors used in systems and more particularly to the automatic calibration of redundant sensors (more than one set of sensors sensing the same conditions).
2. Description of the Prior Art
Many systems employing sensors such as inertial sensors (gyros and accelerometers) used in aircraft, have a need for automatically calibrating the sensors from time to time to remove the effects of accumulated drift over a period of time. In aircraft applications, three gyros and three accelerometers are usually used to sense the rotational and acceleration changes that occur during a flight. A computer then operates to determine the aircraft attitude, position and velocity from these changes.
In order to prevent errors in these readings, it has been the custom to calibrate the sensors when position and velocity are known such as at the airport terminals. When the computer knows the real position and velocity (as it does when the aircraft is at the terminal), it can determine which, if any, of the sensors have drifted and can produce a compensating signal to calibrate the erroneous sensor. Such a prior art system is shown in FIG. 1.
In FIG. 1, a set of three gyros labelled G1, G2 and G3 and identified by reference numerals 11, 13 and 15 are shown providing outputs on lines 17, 19 and 21 through summing circuits 23, 25 and 27 and lines 29, 31 and 33 to e. Navigation Processor 35.
Similarly, in FIG. 1, a set of three accelerometers labelled A1, A2 and A3 and identified by reference numerals 37, 39 and 41 are shown providing outputs on lines 43, 45 and 47 through summing circuits 49, 51 and 53 and lines 55, 57 and 59 to the Navigation Processor 35.
Navigation Processor 35 operates on the signals from Gyros 11, 13 and 15 and from the accelerometers 37, 39 and 41 to produce a signal indicative of latitude on a line 61, a signal indicative of longitude on a line 63, a signal indicative of altitude on a line 65, a signal indicative of north velocity on line 67, a signal indicative of east velocity on line 69, and a signal indicative of vertical velocity on a line 71, which signals are presented through summing circuits 73, 75, 77, 79, 81, and 83 and on lines 85, 87, 89, 91, 93 and 95 respectively to a box identified as Navigation Auto Calibration box 100 as well as to any desired aircraft indicators (not shown). It should be understood that while Navigation Processor 35 and Navigation Auto Calibration box 100 have been shown as two separate boxes for purposes of explanation, they may be combined in a single overall processor with a computer enclosed in a single box.
Summing circuits 73-83 are shown receiving additional inputs from terminals 101, 103, 105, 107, 109 and 111 via lines 113, 115, 117, 119, 121 and 123 respectively. When the aircraft is of known positions and velocities (such as at airport terminals), signals indicative of known latitude, longitude, altitude, north velocity, east velocity and vertical velocity respectively are presented to the terminals 101-111 and are combined in summing circuits 73-83 so that any differences or errors between the outputs from Navigation Processor 35 on lines 61-71 are presented to Navigation Auto Calibration box 100. From these inputs the Navigation Auto Calibration box 100 computes which, if any, of the gyros and accelerometers are in error, and operates to produce corrective signals, if any, on lines 125, 127, 129, 131, 133 and 135, which signals are fed back to the summing circuits 23, 49, 25, 51, 27 and 53 respectively so as to add the corrective signals to the erroneous signals from the gyros and accelerometers and thus provide the desired corrective calibration.
The prior art system of FIG. 1 is satisfactory for use when no more than 1 set of sensors (in this case, 3 gyros and 3 accelerometers) are used. However, redundant systems employ more than 1 set of sensors (in the present example this would be four or more gyros and/or accelerometers) and the prior art does not receive enough information in the arrangement shown in FIG. 1 to allow a determination of which specific gyros and accelerometers need corrective calibration.